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United States Patent |
5,132,940
|
Culbert
|
July 21, 1992
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Current source preamplifier for hydrophone beamforming
Abstract
A preamplifier for beamforming hydrophone signals of a sonar array. The
invention uses an amplifier that provides a current-source type of output
for driving the beamforming network, thus avoiding the substantial signal
loss inherent in using voltage-source amplification. To obtain the
stability of gain required in this type of operation, the invention
employs a negative feedback system that can be easily adjusted by choice
of resistors to accommodate desired levels of shading and signal
equalization. The advantages provided are savings in cost, reduction in
size and power requirements and improved in signal-to-noise ratio.
Inventors:
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Culbert; James A. (Hingham, MA)
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Assignee:
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Hazeltine Corp. (Greenlawn, NY)
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Appl. No.:
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715352 |
Filed:
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June 14, 1991 |
Current U.S. Class: |
367/135 |
Intern'l Class: |
H04B 001/06 |
Field of Search: |
367/135,103
330/98,102,103
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References Cited
Other References
Urick, Principles of Underwater Sound, 1983, p. 32.
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Primary Examiner: Pihulic; Daniel T.
Attorney, Agent or Firm: Onders; E. A.
Claims
What is claimed is:
1. A current-source preamplifier for amplifying a hydrophone output signal
comprising:
first amplification means for amplifying the hydrophone output signal, said
first means having inverting and non-inverting input ports for receiving
the output signal of said hydrophone and said first means having an output
port for providing an amplified signal corresponding to the input signal
provided at the non-inverting input port;
second amplification means for amplifying the output signal provided by
said first means, said second means having an input for receiving the
output signal from said first means and said second means having a
plurality of output ports, at least one of said output ports providing an
amplified current-source type of output corresponding to the input
received from said first means.
2. The current-source preamplifier of claim 1 wherein said second
amplification means further comprises:
feedback means for stabilizing the output gain said preamplifier from
fluctuations in output gain resulting from temperature changes and
component variations of said preamplifier, wherein said feedback means
resistively couples an output port of said second means to an input port
of said first means.
3. The current-source preamplifier of claim 2 wherein said second means
further comprises a transistor in which the base is connected to the
output port of said first means and the collector of the transistor
provides an amplified output signal to a beamforming network.
4. The current-source preamplifier of claim 3 wherein said feedback means
further comprises a resistive path from the emitter of the transistor of
said second means connected to the inverting input port of said first
means.
5. The current-source preamplifier of claim 2 wherein said second means
further comprises an output port connected to at least one on-axis beam
summing network.
6. The current-source preamplifier of claim 5 wherein said feedback means
further comprises a resistive path from an output port connected to an
on-axis beam summing network to a reference direct current potential, said
feedback means stabilizing the output signal provided to the on-axis
summing network.
7. The current source preamplifier of claim 6 wherein said second means
further comprises a transistor in which the base is connected to the
output port of said first means, the collector is connected to a
beamforming network and the emitter is connected to at least one on-axis
summing network.
8. The current-source preamplifier of claim 7 wherein feedback means
further comprises a resistive path from the emitter of the transistor of
said second means connected to the inverting input port of said first
means and a resistive path from the emitter of the transistor of said
second means connected to a reference direct current potential.
9. A sonar array network having a plurality of hydrophones comprising:
a plurality of current-source type of output preamplifiers for each of the
hydrophones, each preamplifier having at least one output port that
provides a feedback stabilized amplified signal corresponding to each
hydrophone in said array;
at least one beamforming network, each beamforming network having a
plurality of input channels for each preamplifier, each input channel of
said beamforming network connected to the feedback stabilized output of
said preamplifiers; wherein the output of said beamforming network is
representative of a sonar beam pattern;
each said preamplifier further comprising at least one resistor for each
output port, independently selectively, corresponding to the output gain
of its associated output port;
each said channel of said beamforming network further comprising at least
one resistor, independently selectable, corresponding to said channel's
signal output; wherein signal outputs from each preamplifier and from each
channel of the beamforming network are shaded independently.
10. The sonar array in claim 9 further comprising:
at least one summing network, each summing network having a plurality of
input channels for each said preamplifier, each input channel of said
summing network connected to a feedback stabilized output port of each
said preamplifier, each output port of said preamplifier being separate
from the output port that is connected to a channel of said beamforming
network;
said summing network further comprising a resistor for each said input
channel, independently selectable, corresponding to the output gain of its
associated preamplifier wherein signal outputs from each preamplifier and
its corresponding on-axis summing network channel are shaded independently
from one another and are shaded independently from the shading provided in
the beamforming network.
11. The sonar array in claim 10 wherein the preamplifiers, beamforming
networks, and on-axis summing networks can be resistively adjusted so that
the output impendance of said beamforming networks and said on-axis
summing networks are substantially equal.
12. The sonar array in claim 11 wherein the outputs of the beamforming
networks and the outputs of the on-axis summing networks are directly
connected to a multiplexing network.
13. A method of providing output signals from multi-channel beamforming
networks and from multi-channel on-axis summing networks that can be
connected directly to the multiplexer network of a sonar array comprising
the steps of:
amplifying the input signals to said beamforming networks through
current-source amplification;
resistively stabilizing the input signals to said beamforming networks
using a negative feedback system that utilizes resistive elements in said
beamforming networks;
shading the input signals to said beamforming networks to produce a signal
representative of the sonar beam pattern of said array;
amplifying the input signal to said on-axis summing networks;
resistively stabilizing the input signals to said on-axis summing networks
incorporating at least part of the negative feedback system used to
stabilize the input signals to said beamforming networks and utilizing
resistive elements in said on-axis networks;
adjusting through resistance selection the input signals to said
beamforming networks and said on-axis networks so that the output
impendance of said networks are substantially equal and corresponds to the
load of the multiplexer of said sonar array.
14. The method of claim 13 wherein the step of shading further comprises
the step of:
selecting proportionate resistance values in each channel of said
beamforming networks that corresponds to the amplified input signal to
each channel wherein a beam pattern is provided at the output of said
beamforming network having a major lobe whose direction is at a
predetermined angle with respect to a line representative of the axis of
the sonar array.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to sound and navigation ranging (sonar) arrays
employing a solid state preamplifier apparatus which provides improved
signal to noise ratio performance, and reductions in cost, required space
and power consumption.
2. Description of the Related Art
As is well known by those skilled in sonar technology, hydrophones are
normally used in arrays in which a number of spaced elements are employed.
Such arrays have several advantages over a single hydrophone, namely,
greater sensitivity, directional properties, and better signal-to-noise
ratio. (Robert J. Urick, Principles of Underwater Sound, (3rd ed.; New
York, McGraw Hill Book Company, 1983), p. 32. Typically, each hydrophone
in any array is followed by a separate preamplifier channel in which
amplification and shading functions are performed. These shading
functions, used to determine the shape of the sonar beam pattern, are well
understood by those skilled in the art. The outputs of the preamplifier
channels of the array are combined in beamforming apparatus to form one or
more sonar beams. Since sonar arrays may consist of anywhere from two to a
large number of hydrophones, economy of space, cost, and power usage are
important factors in the design process. Because of the low power output
of hydrophones, preamplifier voltage gains of the order of 40 dB have been
used. However, the manner in which prior-art preamplifiers have been
coupled to the analog beamforming apparatus has resulted in significant
attenuation of the signal at the beamformer. Specifically, most
preamplifiers have voltage-source outputs, whereas the beamforming
apparatus has required current-source drive. To effect conversion of the
preamplifier voltage source to a current source, a resistor whose
resistance was high in comparison to the beamformer input impedance was
used to couple each preamplifier output to the beamformer, a technique
well-known to circuit designers. While effective for the described
conversion, the technique results in significant undesired attenuation of
the signals at the beamformer's inputs.
Two disadvantages result from the aforementioned undesired attenuation.
First, subsequent amplification after the beamformer has been required to
compensate for the attenuation. Second, the attenuated signals at the
beamform have been at such a low level that pickup of even low levels of
undesired electromagnetic interference (EMI) on the beamformer and its
connecting leads has been sufficient to degrade the signal-to-noise (S/N)
ratio. Shielding and other measures to reduce the EMI result in increased
cost.
Simplified preamplifiers which have current-source outputs, with feedback
stabilized gain, which therefore may be coupled directly to the
beamforming apparatus without causing the aforementioned attenuation is
not found in prior-art apparatus.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a current source preamplifier
that eliminates the need for buffer amplifiers to couple the beamformer
and on-axis summer outputs.
It is a further object of the invention to provide a current source
preamplifier that improves signal to noise ratio performance.
It is still another object of the invention to provide a current source
preamplifier that reduces the number of parts, cost, space and power
consumption as compared to prior art devices.
Finally, it is an objection of the invention is to provide a current source
preamplifier that, through proportioning of parameters of the invention
and summing apparatus, makes it possible:
1. shading the signal produced by the hydrophone array;
2. equalization of the beamformer and on-axis summers, and the outputs of
the beamformer and on-axis summers, and
3. equalization of the output impedances of said beamforming and on-axis
summing apparatus so as to properly match the input-impedance requirements
of multiplexing apparatus to which the beamformer and on-axis summer
outputs are coupled.
The invention is a current-source preamplifier for amplifying a hydrophone
output signal. First amplification means for amplifying the hydrophone
output signal is provided. Said first means has inverting and
non-inverting input ports for receiving the output signal of said
hydrophones. Also, said first means has an output port for providing an
amplified signal corresponding to the input signal provided at the
non-inverting input port which is a voltage source output. Second
amplification means for amplifying the output signal provided by said
first means is provided. Said second means has an input for receiving the
output signal from said first means and said second means has a plurality
of output ports. At least one of said output ports provides an amplified
current-source type of output corresponding to the input received from
said first means. Feedback means for stabilizing the output gain of said
preamplifier from fluctuations in output gain resulting from temperature
changes and component variations of said preamplifier is provided, wherein
said feedback means resistively couples an output port of said second
means to an input port of said first means.
For a better understanding of the present invention, together with other
and further objects, reference is made to the following description, taken
in conjunction with the accompanying drawings, and its scope will be
pointed out in the appended claims.
BRIEF DESCRIPTION OF DRAWINGS
FIG. 1 is a circuit diagram, partially schematic and partly in block form,
of a portion of prior-art sonar array apparatus.
FIG. 2 is a circuit diagram, also partially schematic and partially in
block form, of a portion of sonar array apparatus incorporating the
present invention.
DETAILED DESCRIPTION OF THE INVENTION
In the following discussion, the term beamformer, or beamformer apparatus,
will refer to apparatus which forms a beam at some angle other than the
normal to the plane of the hydrophone array, or, in a line array, to the
line on which the hydrophones lie. The beamformer may form beams that are
at both positive and negative angles with respect to the said normal.
Additionally, the preamplifier outputs may also be coupled to summing
apparatus to form one or more on-axis beams, i.e., beams whose directions
are along the normal to the plane, or line, of the hydrophone array. (Two
on-axis beams are commonly formed, one wide and one narrow).
In sonar system design the usual practice is to improve system performance
through the use of several hydrophones arranged in an array which may
consist of the several hydrophones spaced along a straight line. As is
well known to those skilled in the art, the summed output of all the
hydrophones provides directivity in the reception of a desired signal of
periodic nature. Maximum amplitude of the received signal, i.e., maximum
sensitivity, occurs when the signal source lies somewhere along the array
axis, i.e., a line normal to the center of the line through the array
hydrophones. Further, the summed output from the array has better
signal-to-noise (S/N) ratio than the output from one hydrophone alone, if
the noise is stochastic in nature, being uncorrelated and coming equally
from all directions to all hydrophones.
Normally the distance between adjacent hydrophones in a line array is
one-half the wavelength of the received signal. As is known to those
skilled in the art, as the signal source moves off to either side of the
hydrophone-array axis, the signal thereby comes to the array at some angle
to the axis, the periodic nature of the signal will cause the sum of the
hydrophone outputs to maximize at more than one angle of reception. Thus,
the reception pattern of the array is essentially a beam pattern in which
the center of the main lobe lies along the hydrophone-array axis, and side
lobes are positioned at specific angles either side of the main lobe. The
magnitude of these side lobes relative to the main desired lobe are
reduced by reducing the amplitudes of the electrical signals from the
hydrophones toward the ends of the array (relative to the amplitudes of
the electrical signals from the hydrophones at or near the center of the
array) prior to summing the signals. This technique is known by the term
shading, and reduces the amplitude of the side lobes that are generated
when hydrophones are summed.
As is well known to those skilled in the art, the main lobe of the beam
pattern of a sonar array can be steered to a specific angle relative to
the array axis by incorporating predetermined time delays or phase shifts,
in the paths of the electrical signals detected by the hydrophones in an
array. Typically there is no delay or phase shift applied in the path of
the hydrophone at one end of the steered array, but delay or phase shift
is added in each hydrophone signal path, successively increasing by some
predetermined increment proceeding down the array. A common procedure,
using time delay, is to couple the preamplified signals from successive
hydrophones along the array to corresponding successive taps along a delay
line, each tap separates the signal from adjacent taps by some
predetermined incremental time delay. This is the technique employed in
the embodiments described herein, but the features of the invention are
also applicable in apparatus employing phase shift, rather than time
delay.
In sonar apparatus where sufficient power and space are available digital
signal processing techniques are normally used to perform the beamforming
and shading functions. However, where space and power are at a premium,
and where few beams are needed, it is advantageous to use analog
techniques. The apparatus covered by the present invention falls in the
latter category. The embodiments described use a six-hydrophone array, but
the features of the invention are by no means restricted to that number
and are equally applicable to both larger and smaller arrays.
To provide a basis for better describing the features of the invention, one
form of prior-art apparatus will be described first. FIG. 1 shows the
circuit diagram (partially schematic and partially block) of that portion
of a six-channel sonar array which is pertinent to the application of the
present invention. For the sake of clarity, only one signal channel,
hydrophone 1 plus preamplifier 10, is shown. The outputs of the remaining
five channels (not shown) would similarly be connected to terminals 22
through 26 of the beamformer input-resistor network 20, terminals 42
through 46 of the on-axis narrow-beam summer 40, and terminals 62 through
66 of the on-axis narrow-beam summer 60.
Signals picked up by hydrophone 1 are coupled to the input of preamplifier
10 where the signal is first limited in low-level limiter 11 which
protects the following amplifier apparatus from damage otherwise caused by
very high voltage surges which may result from explosive shock, for
example. The limiter output is resistively coupled to differential
amplifier 12, where, in the embodiment described, the signal is amplified
of the order of 20 dB. The output of differential amplifier 12 is coupled
to the non-inverting input of operational amplifier (opamp) 13, where the
signal is further amplified of the order of 20 dB. As is well known to
those skilled in the art, the resistance values of resistors 14 and 15,
relative to each other, determine the gain of opamp 13. It will be noted
that for simplification of the diagram, required power supply and possible
bias connection, which would be well-known to those skilled in the art,
have not been shown.
The output of preamplifier 10 appearing at terminal 14 is coupled to input
terminal 81 of beamformer 80 through resistor 27 in beamformer
input-resistor-network 20. Beamformer 80 contains a tapped delay line, and
input terminals 81 through 86 are coupled to connections along the length
of the line, as is well known to those skilled in the art.
The output of preamplifier 10 at terminal 14 is a voltage-source type of
output at low impedance. By comparison the input impedance to the delay
line tap at terminal 81 is relatively high, of the order of 1,500 ohms in
the apparatus described. If preamplifier output terminal 14 were connected
directly to input terminal 81 of the beamformer, the low preamplifier
output impedance would essentially short to ground the delay line tap at
terminal 81, and thereby preclude the desired beamforming function.
Coupling resistor 27, whose resistance is high in comparison to the
delay-line impedance, essentially converts the voltage source at terminal
14 to a current source at terminal 81, thereby permitting beamformer 80 to
perform its desired function. However, as will be obvious to those skilled
in the art, the signal at terminal 81 is significantly attenuated with
respect to the signal at terminal 14. This attenuation has at least two
undesired effects. First, amplification following the beamformer must be
added to compensate for the attenuation. Second, the signals at the inputs
and outputs of beamformer 80 have been reduced to such a low level that
EMI pickup on the connecting leads thereto degrades the S/N ratio. To
alleviate this problem, shielding and other EMI reduction techniques are
required to maintain the integrity of the desired signal. These corrective
measures add cost and bulk to the apparatus.
As discussed earlier, five additional hydrophones and their associated
preamplifiers (not shown in FIG. 1) are included in the array and are
coupled to the appropriate aforementioned input terminals of beamformer
input-resistance network 20 and on-axis summers 40 and 60. By methods to
be described shortly, the signals from the six channels are shaded to
provide electrical signals representative of the desired beam patterns at
the output of the apparatus.
Also, by methods known to persons skilled in the art, delay-line apparatus
(and connections thereto) in beamformer 80 have been arranged to provide
two outputs at terminals 87 and 88, one representing a sonar beam whose
major lobe is at a positive angle with respect to the array axis, and one
with the major lobe at a negative angle. Buffer amplifiers 89 and 90
amplify these two beamformer output signals sufficiently to at least
compensate for the aforementioned attenuation, and the buffer amplifier
outputs are then coupled to the input of multiplexer 93.
Returning to the output of preamplifier 10 at terminal 14, the signal at
that point, in addition to being coupled to the beamformer as just
described, is also coupled to input terminals 41 and 61 of on-axis
wide-band summer 40 and on-axis narrow-band summer 60, respectively. In
these two summers, the preamplifier outputs of all six array channels are
added to produce the desired on-axis beam patterns, using shading
techniques to be described shortly.
The on-axis summing functions are not accompanied by the severe attenuation
inherent in the coupling to beamformer 80. However, the output impedances
of summers 40 and 60 are significantly higher than the output impedances
of buffer amplifiers 89 and 90. Moreover, multiplexer 93, in conjunction
with a following line-driver amplifier (fed from terminal 94, but not
shown in FIG. 1), requires essentially equal impedances of the four
driving sources coupled to its inputs. Therefore, buffer amplifiers 91 and
92 are required following summers 40 and 60, respectively, so as, at a
minimum, to equalize the output impedances of the four beam sources
coupled to the inputs of multiplexer 93, and may be further required by
gain adjustment, to equalize the sensitivities of the four beams.
With respect to shading, a combination of measures was employed to obtain
the desired beam patterns. First, the gains of all six preamplifiers were
not the same. The gains of the preamplifiers in the array, relative to
each other, were set to the predetermined values (not all channels the
same) by separately proportioning resistors 14 and 15 in each array
channel. Further shading adjustments for the outputs of beamformer 80 were
made by the use of differing values of predetermined resistance of the
coupling resistors in beamformer input-resistance network 20.
Similarly, different values of resistance of the resistors in on-axis
wide-band summer 40 were predetermined to partially provide the desired
wide-beam pattern, and yet another set of differing predetermined
resistance values was used in on-axis narrow-beam summer 60.
The prior-art apparatus that has been described was used in an environment
where the desired signal frequency was of the order to 25 kilohertz. The
invention was also applied in an embodiment used in that frequency range,
but the invention features are not restricted to use at that frequency.
FIG. 2 shows the circuit diagram, partially schematic and partially block,
of a portion of a sonar array in which the invention is embodied (the same
portion as shown in FIG. 1). Like the prior-art embodiment of FIG. 1, for
simplicity the power supply connections are not show, and only one of the
array's hydrophones and its accompanying preamplifier are shown.
Connections to the beamformer and summers for a six-channel array are
shown, but the invention features and advantages are equally applicable in
arrays with more or fewer channels.
In the invention embodiment of FIG. 2, signals picked up by hydrophone 1
are coupled to the input of preamplifier 100. As in the prior-art
apparatus, the signal is limited in low-level limiter 101 and is then
coupled to the non-inverting input of opamp 102. The opamp output is then
coupled to the base of transistor 103. The AC signal voltage across
emitter resistor 104 appearing at terminal 109 is very nearly equal to the
signal voltage at the base of the transistor, and is essentially of the
same phase. The signal at the collector output terminal 108 is of opposite
phase with respect to the phase at the transistor base and emitter.
Resistors 106 and 105 form a negative feedback loop from the emitter of
transistor 103 to the inverting input of opamp 102. The manner in which
preamplifier 100, employing the aforementioned feedback loop, uniquely
provides for the features of the invention will be described shortly.
In the preferred embodiment of the invention, transistor 103 was a low-cost
general purpose bipolar junction transistor (Motorola type 2N4124).
However, the features of the invention are not confined to the use of
bipolar junction transistors, and may be obtained with other types, such
as CMOS or field-effect transistors, for example. The transistor replaces
an expensive low-noise opamp having a high grain-bandwidth product which
was used in the FIG. 1 prior-art apparatus.
One of the desired features in this invention was to provide a
current-source output at terminal 108 of each preamplifier for driving
each of the inputs (80 through 86) of beamformer 80, thereby avoiding the
substantial signal loss at the beamformer inherent in the prior art
apparatus. This objective alone could have been achieved with a
preamplifier configured as unit 100 in FIG. 2 but with feedback resistors
105 connected to the base of transistor 103 rather than the emitter, a
normal opamp feedback connection. As is known by those skilled in the art,
a transistor amplifier with an unbiased emitter resistor will exhibit the
characteristic of a current-source at its collector. However, the gain of
the transistor in such an arrangement will not meet the stability
requirements of a sonar array. There will be unacceptable variations in
gain caused by variations in the dc current gain (h.sub.FE) and
base-to-emitter voltage (V.sub.BE) of the transistor due to both
temperature changes and variations from one device to another within
normal production tolerance ranges for the transistor device being used.
In sonar arrays, stability of the gains of the preamplifiers is highly
important in order to maintain the shading relationships required to
preserve the beam-shape integrity, and further to maintain equality (or
other required relationship) of the peak sensitivities of the various
beams formed in the sonar apparatus. The coupling of the feedback loop
from the emitter of transistor 103 through the network of resistors 105
and 106 to the inverting input of opamp 102 not only provides simple,
economical, unique, and versatile means for meeting the shading and
equality of beam-sensitivity requirements of the array.
In FIG. 2, the preamplifier gain from the non-inverting input of opamp 102
to emitter output terminal 109 of transistor 103 is essentially determined
by the proportioning of resistors 105 and 106 in the feedback loop, as is
well understood by those skilled in the art. This feedback coupling
essentially eliminates the undesired gain changes to emitter terminal 109
that would otherwise occur due to the previously mentioned undesired
variations in V.sub.BE and h.sub.FE. It will also be understood by those
skilled in the art that the preamplifier gain to emitter terminal 109 is
essentially independent of the resistance value of emitter resistor 104.
However, the gain to collector output terminal 108 is additionally a
function of the resistance value of emitter resistor 104. Thus, to
summarize, the preamplifier gain to output terminal 109 is essentially a
function of the relative resistance values of feedback network resistors
105 and 106, whereas the gain to output terminal 108 is a function of both
the feedback network resistances and the resistance of emitter resistor
104.
Referring to beamformer 80, typically the input signal amplitudes to the
six beamformer inputs in the embodiment of FIG. 2, would be shaded. The
signal levels at the two ends, terminals 81 and 86, may, for example, be
of the order of one-half the level at the center two terminals, 83 and 84,
and the signal levels at terminals 82 and 85 may be of the order of 0.7
times the level at the center two terminals. However, these figures are
given only as an example for assistance in understanding the operation of
the apparatus, and are not critical with respect to the unique features of
the invention. In order to provide shading at the beamformer, such as in
the example above, the gains to output terminals 108 of the six
preamplifiers in the array, relative to each other, may be adjusted in a
variety of ways as suggested previously. The relative gains of the six
channels may be set to the values required for shading by proportioning
the feedback network resistors 105 and 106 separately for each channel,
or, by selecting the resistance value of emitter resistor 104 in each
channel, or by a combination of both methods. The latter use of combined
methods was employed in the embodiment of FIG. 2.
Looking now at the on-axis beam summers 200 and 400, again there is a
variety of ways to achieve the on-axis beam shading. First, as has been
described, the relative preamplifier gains to emitter output terminals 109
may be set to predetermined values by proportioning of resistors 105 and
106 in the feedback networks, or shading may be achieved through design of
the resistor networks in summers 200 and 400. A combination of both
methods was used in the preferred embodiment.
In addition to shading requirements, summer networks 200 and 400 are also
designed to provide output impedances that are essentially the same as the
output impedance of the beamformer. The equalization was required by the
multiplexer in conjunction with its following line amplifier (not shown)
in the embodiment of FIG. 2. The summer networks do not necessarily need
to be in the form shown in blocks 200 and 400 to meet the requirements of
the invention apparatus. The essential requirements of the summer networks
are that 1) in conjunction with predetermined gains of the six
preamplifiers, 100, the networks shall provide the desired shading, 2)
each summer's output amplitude at the maximum sensitivity point of its
beam shall be essentially equal to the maximum output of beamformer 80 at
the maximum sensitivity point of the beam pattern it represents, and 3)
the output impedances of the two summers shall be essentially the same as
the output impedances of the beamformer. An example of a possible simple
variation in networks 200 and 400 falling within the spirit of the
invention would be the addition of a resistor to ground at the output of
each summer network, if required.
Thus, it is apparent that the invention uniquely provides several methods
which may be used separately in some instances, or in combination to
provide shading for multiple beams, equalized beam amplitudes and
equalized output impedances at the outputs of beamformer 80 and on-axis
summers 200 and 400. It will be evident to those skilled in the art that
there is a wide choice of combinations of methods for varying gains and
combining signals which will provide the features of the invention. A
guiding principle in the design of a preferred embodiment of the invention
was to use combinations of methods which permitted the use of resistors in
the feedback networks, 105 and 106, the emitter resistor 104, and summing
networks 200 and 400 which were at or near standard values of resistance.
If the on-axis beams are not required in the sonar apparatus,
simplification of the circuit of FIG. 2 may be accomplished while at the
same time retaining features of the invention. On-axis summer networks 200
and 400 may simply be deleted, or additionally emitter resistor 104 may be
omitted and the emitter of transistor 103 may be grounded. For the latter
case, feedback resistor 105 would be reconnected to the collector of the
transistor, and output signal from limiter 101 would be coupled to the end
of resistor 106 shown as the grounded end in FIG. 2. Necessary bias
changes and grounding procedures are known to those skilled in the art.
The output of beamformer 80 would also be reversed.
In summary, features provided by the invention include:
1. A preamplifier with current-source output to the beamformer which is not
accompanied by the gain loss inherent in prior-art apparatus. In the
preferred embodiment the delay-line impedance of beamformer 80 was
approximately 1500 ohms, and the collector impedance of transistor 103 was
greater than 1 megohm. The gain of preamplifier 100 to terminal 108 was of
the order of 40 dB. However, these impedance and gain values are not
critical with respect to the advantages of the invention.
2. The use of and the opamp plus a low-cost transistor in preamplifier 10
results in a cost saving compared to prior-art apparatus which used two
expensive low-noise high gain bandwidth product opamps, and also results
in a reduction in required space and power.
3. Because the preamplifier provides a much higher level of signal to the
beamformer than prior-art apparatus, the signal-to-noise ratio is not
degraded by EMI pickup at the beamformers, and costly shielding and EMI
reduction measures are not required.
4. The buffer amplifiers following the beamformer in prior-art apparatus
have been eliminated with substantial savings in cost and a reduction in
required space and power.
5. Because the output impedances of summer networks 200 and 400 can be
equalized to the output impedance of beamformer 80, buffer amplifiers
following the summers are not required as they were in the prior-art
apparatus, effecting further savings in cost and a reduction in required
space and power.
Although the invention embodiment shown in the diagram of FIG. 2
incorporates one beamformer with two outputs, and two on-axis summers, the
invention is not limited to that number of beamforming and summing
apparatus.
While there have been described what are at present considered to be the
preferred embodiment of this invention, it will be obvious to those
skilled in the art that various changes and modifications may be made
therein without departing from the invention and it is, therefore, aimed
to cover all such changes and modifications as fall within the true spirit
and scope of the invention.
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